1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device having a structure in which, for example, a PNP transistor and an NPN transistor are formed on a semiconductor substrate, and more specifically to a method of manufacturing a so-called polycrystalline silicon emitter transistor (poly emitter transistor), in which an emitter region of a bipolar transistor is formed by diffusion of an impurity from the polycrystalline silicon film into the substrate.
2. Description of the Related Art
Conventionally, in a so-called poly emitter transistor, in which an emitter region of a bipolar transistor is formed by diffusing impurities from the polycrystalline silicon film into the substrate, the ion implantation method is generally used for the introduction of an impurity into the polycrystalline silicon film.
Examples of the impurity introduced by the ion implantation method are arsenic, in the case of an NPN transistor, and boron (B) or boron difluoride (BF.sub.2) in the case of a PNP transistor.
As the size of a bipolar transistor is decreased in the downsizing trend, the width of its emitter region is also reduced. Along with this, the aspect ratio of the emitter hole (height of emitter hole with respect to the width of emitter) is increased.
FIGS. 1A and 1B are diagrams showing the case where an impurity 4 is introduced by ion implantation into a polycrystalline silicon film 3 deposited on an insulating film 2 formed on a semiconductor substrate 1 in the process of manufacturing a bipolar transistor.
In this case, if the aspect ratio of an emitter hole 5 made in the insulating film 2 is large, a section where the impurity 4 is not introduced during the ion implantation, is created in the side wall portion of the emitter hole 5, as shown in FIG. 1A, or in the polycrystalline silicon film 3 of the deepest section as shown in FIG. 1B.
As a result, the concentration of the impurity in the polycrystalline silicon film 3 is not uniformed as shown in FIG. 2, and consequently, thus formed emitter region 6 has a distorted shape. Therefore, a non-uniformity of the characteristics of the bipolar results, or the characteristics are degraded.
In order to solve this problem, there has been an attempt in which a polycrystalline silicon film in which an impurity has been doped in advance is deposited. For example, in the case of an NPN transistor, such an attempt is the method of depositing a polycrystalline silicon film in which phosphor has been doped.
With this method, however, only one of two conduction types of polycrystalline silicon film can be deposited.
When emitter regions having conduction types different from each other are formed by diffusing an impurity from the polycrystalline film so as to manufacture an NPN transistor and a PNP transistor at the same time, there is only one possible method of introducing an impurity, and that is the ion implantation method. With the ion implantation method, however, the variance of the characteristics of the transistors due to the downsizing thereof is not inevitable, creating a problem.